[Haskell-cafe] New New newbie question/help
Paul Hudak
paul.hudak at yale.edu
Wed Jun 27 08:02:56 EDT 2007
I think that an easier solution is to just change the type of
equilateralTri to:
equilateralTri :: Window -> Float -> Float -> Float -> IO()
But I am on the road and wasn't able to actually run the code.
-Paul
Dougal Stanton wrote:
> On 27/06/07, Balu Raman <braman09 at gmail.com> wrote:
>
>> equilateralTri :: Window -> Int -> Int -> Int -> IO()
>> equilateralTri w x y side
>> = drawInWindow w (withColor Red
>> (polygon
>> [(x,y),(a,b),(x,y)]))
>> where
>> b = y + side * sin(pi/3)
>> a = x + side * cos(pi/3)
>
> Your problem lies in this section here. Let's look at the error message:
>
>> triangle.hs:17:36:
>> No instance for (Floating Int)
>> arising from use of 'pi' at triangle.hs:17:36-37
>> Probable fix: add an instance declaration for (Floating Int)
>> In the first argument of '(/)', namely 'pi'
>> In the first argument of 'cos', namely '(pi / 3)'
>> In the second argument of '(*)', namely 'cos (pi/3)'
>> Failed, modules loaded: none
>
> The problem comes from the calculations of 'a' and 'b'. The function
> sin doesn't return an Int value. It returns types within the type
> class Floating (annotated as below, for some unspecified 'a').
>
>> sin (pi/3) :: Floating a => a
>> side :: Int
>
> Since the type checker has one unknown type, a, and one known, Int, it
> tries to put the two together. Then it finds that Int is not an
> instance of the Floating class, so a /= Int. So it asks you to make
> one:
>
>> Probable fix: add an instance declaration for (Floating Int)
>
> In this case, the advice is bad. There is no reasonable way of making
> a machine integer a member of the floating class. What you need to do
> instead is ensure that you're using a type that is a member of the
> Floating class - that is, convert from an Int before you start the
> calculation.
>
> The function fromIntegral should come in handy:
>
>> let n = 3 :: Int
>> (fromIntegral n) * sin (pi/3)
> 2.598076211353316
>
>
> Good luck!
>
> D.
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